The present invention relates to a lithographic process of forming a lithographic printing plate. In more detail, the invention relates to a lithographic process of scanning a presensitized lithographic plate with a laser beam according to digital signals to form a lithographic printing plate.
A lithographic printing plate generally comprises a hydrophobic imaging area, which receives oily ink in a printing process, and a hydrophilic non-imaging area, which receives dampening water. A conventional lithographic process usually comprises the steps of masking a presensitized (PS) plate comprising a hydrophilic support and a hydrophobic photosensitive resin layer with a lith film, exposing the plate to light through the lith film, and then dissolving and removing a non-imaging area with a developing solution.
Nowadays a computer electronically processes, stores and outputs image information as digital data. An image according to the digital data is preferably formed by directly scanning a presensitized lithographic plate with a highly directive active radiation such as a laser beam without use of a lith film. The lithographic process of forming a printing plate according to digital image data without use of a lith film is referred to as Computer to Plate (CTP).
The conventional lithographic process of forming a printing plate has a problem about CTP that a wavelength region of a laser beam does not match a spectral sensitivity of a photosensitive resin.
The conventional PS plate requires a step of dissolving and removing a non-imaging area (namely, developing step). The developed printing plate should be further subjected to post-treatments such as a washing treatment using water, a rinsing treatment using a solution of a surface active agent, and a desensitizing treatment using a solution of gum arabic or a starch derivative. The additional wet treatments are disadvantageous to the conventional PS plate. Even if an early step (image-forming step) in a lithographic process is simplified according to a digital treatment, the late step (developing step) comprises such troublesome wet treatments that the process as a whole cannot be sufficiently simplified.
The printing industry as well as other industries is interested in protection of global environment. Wet treatments inevitably influence global environment. The wet treatments are preferably simplified, changed into dry treatments or omitted from a lithographic process to protect global environment.
A process without wet treatments is referred to as a press development method, which comprises the steps of attaching an exposed presensitized printing plate to a cylinder of a printer, and rotating the cylinder while supplying dampening water and ink to the plate to remove a non-imaging area from the plate. Immediately after exposing the presensitized plate to light, the plate can be installed in a printer. A lithographic process can be completed while conducting an usual printing treatment.
A presensitized lithographic printing plate suitable for the press development method must have a photosensitive layer soluble in dampening water or a solvent of ink. The presensitized plate should easily be treated under room light to be subjected to a press development in a printer placed under room light.
A conventional PS plate cannot satisfy the above-described requirements.
Japanese Patent No. 2,938,397 (corresponding to European Patent No. 0770494, and U.S. Pat. Nos. 6,030,750 and 6,096,481) discloses a method for making a lithographic printing plate. The method uses an imaging element (presensitized plate) comprising on a hydrophilic surface of a lithographic based an image forming layer comprising hydrophobic thermoplastic polymer particles capable of coalescing under the influence of heat and dispersed in a hydrophilic binder and a compound capable of converting light to heat. The method comprising the steps of imagewise exposing to light the imaging element; and developing a thus obtained imagewise exposed imaging element by mounting it on a print cylinder of a printing press and supplying an aqueous dampening liquid or ink to the image forming layer while rotating the printer cylinder.
The imaging element can be treated under room light because the element has sensitivity within an infrared region.
In the method for making a lithographic printing plate, polymer particles coalesce under the influence of heat converted from light. Imaging elements having particles suitable for a press development often show poor plate wear. On the other hand, imaging elements showing good plate wear often cause troubles at a press development or stains in a printed sheet. Therefore, it is difficult to form a printing plate improved in plate wear according to a press development.
An object of the present invention is to provide a lithographic printing plate improved in plate wear.
Another object of the invention is to provide a pre-sensitized lithographic printing plate improved in sensitivity and resolution.
The first embodiment of the present invention provides a lithographic process comprising the steps of: imagewise heating a presensitized lithographic printing plate which comprises a hydrophilic support and an image-forming layer containing a hydrophobic polymer and a compound having two or more o-quinodimethane structures or precursor structures thereof to polymerize the compound; and removing the unheated area of the image-forming layer to form a lithographic printing plate.
The first embodiment of the invention also provides a lithographic process comprising the steps of: imagewise heating a presensitized lithographic printing plate which comprises a hydrophilic support and an image-forming layer containing a hydrophilic polymer and a compound having two or more o-quinodimethane structures or precursor structures thereof, said compound being contained in particles dispersed in the hydrophilic polymer to polymerize the compound and to fuse the particles together; and removing the unheated area of the image-forming layer to form a lithographic printing plate.
The first embodiment of the invention further provides a lithographic printing process comprising the steps of: imagewise heating a presensitized lithographic printing plate which comprises a hydrophilic support and an image-forming layer containing a hydrophilic polymer and a compound having two or more o-quinodimethane structures or precursor structures thereof, said compound being contained in particles dispersed in the hydrophilic polymer to polymerize the compound and to fuse the particles together; attaching the plate to a printer; working the printer to remove the unheated area of the image-forming layer and to form a lithographic printing plate; and printing with the formed plate by supplying oily ink and dampening water to the plate.
The first embodiment of the invention furthermore provides a presensitized lithographic printing plate which comprises a hydrophilic support and an image-forming layer containing a hydrophobic polymer and a compound having two or more o-quinodimethane structures or precursor structures thereof.
The first embodiment of the invention still further-more provides a presensitized lithographic printing plate which comprises a hydrophilic support and an image-forming layer containing a hydrophilic polymer and a compound having two or more o-quinodimethane structures or precursor structures thereof, said compound being contained in particles dispersed in the hydrophilic polymer.
The second embodiment of the present invention provides a lithographic process comprising the steps of: imagewise heating a presensitized lithographic printing plate which comprises a hydrophilic support and an image-forming layer containing a hydrophobic polymer having o-quinodimethane structures or precursor structures thereof to cross-link the hydrophobic polymer; and removing the un-heated area of the image-forming layer to form a lithographic printing plate.
The second embodiment of the invention also provides a lithographic printing process comprising the steps of: imagewise heating a presensitized lithographic printing plate which comprises a hydrophilic support and an image-forming layer containing a hydrophilic polymer and a hydrophobic polymer having o-quinodimethane structures or precursor structures thereof, said hydrophobic polymer being contained in particles dispersed in the hydrophilic polymer to cross-link the hydrophobic polymer and to fuse the particles together; attaching the plate to a printer; working the printer to remove the unheated area of the image-forming layer and to form a lithographic printing plate; and printing with the formed plate by supplying oily ink and dampening water to the plate.
The second embodiment of the invention further provides a presensitized lithographic printing plate which comprises a hydrophilic support and an image-forming layer containing a hydrophobic polymer having o-quinodimethane structures or precursor structures thereof.
The lithographic process of the present invention uses a reaction of o-quinodimethane structures contained in a compound (first embodiment) or a polymer (second embodiment). The reaction of the o-quinodimethane structures is hardly inhibited by oxygen in air, while a conventional image forming reaction (such as a polymerization reaction of an ethylenically unsaturated monomer) is remarkably inhibited by oxygen. Therefore, the lithographic process of the invention can form a lithographic printing plate showing excellent plate wear.